I drilled a set of flop and chops that were steel and extremely thick- too thick in fact to fit a reverse lever's plug inside the end of the bar. I used a campy aero lever out at the very end of the bar, and I did some grinding inside the lever to allow the cable and housing to exit towards the bottom of the lever instead of the top like aero levers are designed to do. (I guess this doesn't matter to anyone using reverse levers, but whatever.) I didn't have to cut the actual shell (the part the hood covers) because the housing entered the bar in a spot hidden underneath the lever- I just cut the horizontal brace inside the lever to let the housing cut the angle more gently.
Done this way, on the bar you just put in a notch an inch down from the end with a grinder (I think a notch is better than a drill hole because the housing can pass into the bar at an angle), and another down near the stem. I would be very leery of doing this on anything aluminum- my bars don't feel any different and have held up thus far. Aluminum bars I've used before were very noticeably more flexy than these without being cut. When manufacturers make bars with internal routing, they add internal bracing to get away with using aluminum.
I like the internal routing a lot- this way the bar is symmetrical with just one brake. The housing exits under the wrap an inch up from where it ends, then cleanly sweeps down to the brake. I haven't noticed any increased drag or sticking on the brake cable, from the added bends. I'll get a picture when I have access to a camera.